The present invention is related in general to the field of electronic systems and semiconductor devices, and more specifically to the process of preparing integrated circuit bond pads for wire bonding in copper-metallized ICs.
In integrated circuits (IC) technology, pure or doped aluminum has been the metallization of choice for interconnection and bond pads for more than four decades. Main advantages of aluminum include ease of deposition and patterning. Further, the technology of bonding wires made of gold, copper, or aluminum to the aluminum bond pads has been developed to a high level of automation, miniaturization, and reliability.
In the continuing trend to miniaturize ICs, the RC time constant of the interconnection between active circuit elements increasingly dominates the achievable IC speed-power product. Consequently, the relatively high resistivity, of the interconnecting aluminum now appears inferior to the lower resistivity of metals such as copper. Further, the pronounced sensitivity of aluminum to electromigration is becoming a serious obstacle. Consequently, there is now a strong drive in the semiconductor industry to employ copper as the preferred interconnecting metal, based on its higher electrical conductivity and lower electromigration sensitivity. From the standpoint of the mature aluminum interconnection technology, however, this shift to copper is a significant technological challenge.
Copper has to be shielded from diffusing into the silicon base material of the ICs in order to protect the circuits from the carrier lifetime killing characteristic of copper atoms positioned in the silicon lattice. For bond pads made of copper, the formation of copper(I)oxide films during the manufacturing process flow has to be minimized, since these films severely inhibit reliable attachment of bonding wires, especially for conventional gold-wire ball bonding. In contrast to aluminum oxide films overlying metallic aluminum, copper oxide films overlying metallic copper cannot easily be broken by a combination of thermocompression and ultrasonic energy applied in the bonding process. As further difficulty, bare copper bond pads are susceptible to corrosion.
In order to overcome these problems, a process has been disclosed to cap the clean copper bond pad with a layer of aluminum and thus re-construct the traditional situation of an aluminum pad to be bonded by conventional gold-wire ball bonding. A suitable bonding process is described in U.S. Pat. No. 5,785,236, issued on Jul. 28, 1998 (Cheung et al., xe2x80x9cAdvanced Copper Interconnect System that is Compatible with Existing IC Wire Bonding Technologyxe2x80x9d). The described approach, however, has several shortcomings.
First, the fabrication cost of the aluminum cap is higher than desired, since the process requires additional steps for depositing metal, patterning, etching, and cleaning. Second, the cap must be thick enough to prevent copper from diffusing through the cap metal and possibly poisoning the IC transistors. Third, the aluminum used for the cap is soft and thus gets severely damaged by the markings of the multiprobe contacts in electrical testing. This damage, in turn, becomes so dominant in the ever decreasing size of the bond pads that the subsequent ball bond attachment is no longer reliable.
A low-cost structure and method for capping the copper bond pads of copper-metallized ICs has been disclosed on U.S. patent application Ser. No. 09/775,322, filed on Feb. 1, 2001 (Stierman et al., xe2x80x9cStructure and Method for Bond pads of Copper-Metallized Integrated circuitsxe2x80x9d). A barrier metal layer is deposited over the copper, preferably nickel. For some applications, the nickel layer is followed by a second barrier layer, preferably palladium. The outermost layer is bondable, preferably gold. Based on this metallization sequence, a gold wire bonding method has been described in U.S. patent application Ser. No. 09/817,696, filed on Mar. 23, 2001 (Test et al., xe2x80x9cWire Bonding Process for Copper-Metallized Integrated Circuitsxe2x80x9d).
Especially in the case of electroless nickel, for the control of the deposition process and the structural quality of the deposited layer, a continuous control of the plating bath is necessary. An urgent need has, therefore, arisen for a flexible and reliable method of plating bath control in the nickel plating cycle in order to create the controlled capping of bond pads for subsequent wire bonding. The plating control methods should be flexible enough to be applied for different IC product families and a wide spectrum of design and process variations. Preferably, these innovations should be accomplished while shortening production cycle time and increasing throughput, and without the need of expensive additional manufacturing equipment.
The xe2x80x9cFundamentals of Electrochemical Depositionxe2x80x9d have been described in the book by M. Paunovic and M. Schlesinger (John Wiley and Sons, New York, 1998. Pages 139 and 140 refer to the electroless deposition of nickel. The chemical reactions have been summarized earlier in the xe2x80x9cElectroplating Engineering Handbookxe2x80x9d by Lawrence J. Durney (van Nostrand Reinhold Comp., New York, 1984). Pages pp. 445 through 457 give details of the electroless catalytic nickel deposition specifically for acidic nickel/phosphorus solutions. The interaction between the hypophosphite ion and water leads to the formation of a hydride ion, H-, which then acts to reduce the nickel ions in solution. The atomic hydrogen forms molecules which are subsequently liberated as hydrogen gas. For the nickel plating bath, the equations quoted are:
In acid solution:
H2PO2xe2x88x92+H2Oxe2x86x92HPO3xe2x88x92xe2x88x92+2H++Hxe2x88x92
In alkaline solution:
H2PO2xe2x88x92+2OHxe2x88x92xe2x86x92HPO3xe2x88x92xe2x88x92+H2O+Hxe2x88x92
Further:
Ni+++2Hxe2x88x92xe2x86x92Ni+2H
H++Mxe2x88x92xe2x86x92H2
The acid formulations offer advantages relative to speed of nickel deposition, physical characteristics of the deposit, and stability of the plating bath. With a wide range of organic and inorganic acid salts available, the electroless nickel bath operates as follows:
xe2x80x83Ni+++sodium hypophosphite+(buffers, complexors, accelerators, stabilizers, wetters, moderators)xe2x86x92@catalytic surfacexe2x86x92(Ni+P) deposit+H2+sodium orthophosphite.xe2x80x83xe2x80x83Eq. (*)
It is advantageous for plating nickel uniformly that free hydrogen gas is adsorbed at the surface of the freshly plated nickel. This is best accomplished by intentionally controlling the hydrogen level in the plating bath at a predetermined value.
Besides the temperature of the bath, the factor which influences the nickel deposition rate most is the pH value of the bath. The minimum value for deposition is 3.0, the maximum value is 7.0 at which the hypophosphite spontaneously oxidizes by the reaction:
(H2PO2)xe2x88x92+OHxe2x88x92xe2x80x94(HPO3)xe2x88x92xe2x88x92+H2.
In practice, the preferable pH range is between 4.3 and 4.9.
When the above reactions are applied to the electroplating process in the fabrication of nickel layers as needed in integrated circuit (IC) bond pad structures, the control of the hydrogen level in the plating bath emerged as a critical factor. The present invention discloses a flexible plating method by continuously controlling the hydrogen concentration of the plating bath by adding controlled amounts of hydrogen gas. The control of the hydrogen concentration is provided by
selected distribution and number of nozzles and size of orifices; and
predetermined pressure and duration of hydrogen gas flowing through the nozzles, wherein pressure and duration may be variable with time.
The control of the hydrogen concentration is selected so that the concentration provides a ramp-up phase, needed for a rapid plating start, followed by a saturation phase, needed for consistent plating stability.
With the metal layer plating process under control, the present invention discloses a robust, reliable and low-cost metal structure enabling electrical wire connections to the interconnecting copper metallization of ICs. The structure comprises a layer of barrier metal that resists copper diffusion, deposited on the non-oxidized copper surface. The structure further comprises an outermost bondable metal layer, unto which a metal wire is bonded for metallurgical connection.
The barrier metal is selected from a group consisting of nickel, cobalt, chromium, molybdenum, titanium, tungsten, and alloys thereof. The outermost metal layer is selected from a group consisting of gold, platinum, and silver.
The present invention is related to high density and high speed ICs with copper interconnecting metallization, especially those having high numbers of metallized inputs/outputs, or xe2x80x9cbond padsxe2x80x9d. These circuits can be found in many device families such as processors, digital and analog devices, logic devices, high frequency and high power devices, and in both large and small area chip categories.
It is an aspect of the present invention to be applicable to bond pad area reduction and thus supports the shrinking of IC chips. Consequently, the invention helps to alleviate the space constraint of continually shrinking applications such as cellular communication, pagers, hard disk drives, laptop computers and medical instrumentation.
Another aspect of the invention is to fabricate the bond pad metal caps by the self-defining process of electroless deposition, thus avoiding costly photolithographic and alignment techniques.
Another aspect of the invention is to provide control means from the outside into production processes which need a high degree of flexibility and stability. Specifically, the addition of outside hydrogen gas into electrochemical reactions provides sensitive and immediate control of plating depositions and, to some degree, of the quality of the deposited layer.
Another aspect of the invention is to provide process concepts which are flexible so that they can be applied to many families of semiconductor products, and are general so that they can be applied to several generations of products.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.